The present invention relates to a method and apparatus for measuring the diameter and/or eccentricity of a coating layer of a coated optical fiber and, more particularly, to a method and apparatus that are capable of being utilized to measure the diameter and/or eccentricity of a primary coating layer of a coated optical fiber during an optical fiber cable manufacturing process.
The successful implementation of a light wave communication system requires high quality light guide fibers having mechanical properties sufficient to withstand the stresses to which they are subjected. Each fiber must be capable of withstanding over its entire length a maximum stress level to which the fiber will be exposed during installation and service. The importance of fiber strength becomes apparent when one considers that a single fiber failure will result in the loss of several hundreds of circuits.
The failure of light guide fibers in tension is commonly associated with surface flaws which cause stress concentrations and lower the tensile strength below that of pristine unflawed glass. The size of the flaw determines the level of stress concentration and, hence, the failure stress: Even micron-sized surface flaws cause stress concentrations which significantly reduce the tensile strength of the fibers.
Optical fibers are normally made in a continuous process which involves drawing a thin glass strand of fiber from a partially molten glass preform and thereafter applying the coating layers. A furnace is used to partially melt the preform to permit the fiber to be drawn. The heat of the furnace and the rate of draw of the fiber must be in proper balance so that the optical fiber can be drawn continuously under uniform conditions. Long lengths of light guide fibers have considerable potential strength, but the strength is diminished by airlines or holes occurring in the optical fibers. Furthermore, airlines in optical fibers also interfere with the light-propagation properties of the optical fibers.
Soon after an optical fiber is drawn, the optical fiber is coated with a primary layer of coating material and a secondary layer of coating material. The primary layer of coating material surrounds the optical fiber and serves as a soft cushion for the optical fiber to prevent micro-bending losses. The primary coating layer also seals the outer surface of the optical fiber from environmental conditions, such as atmospheric moisture. This secondary coating layer surrounds the primary coating layer. The secondary coating layer is harder than the primary coating layer and serves to shield the fiber from surface abrasion, which could occur as a result of subsequent manufacturing processes and handling during installation. The secondary coating layer also provides protection against corrosive environments and atmospheric moisture. U.S. Pat. Nos. 5,880,825, 5,828,448 and 5,786,891, which are incorporated herein by reference, are directed to detecting, and/or distinguishing between, defects in an optical fiber coating.
The primary and secondary coating layers are often applied by a dual coating applicator during the fiber drawing process. Therefore, the dual coating applicator applies both the secondary and primary coating layers. The coating layers are subsequently cured in an ultraviolet (UV) lamp system as the coated optical fiber is drawn through the UV lamp system. It is desirable to use a dual applicator for applying both of the coating layers because less coating material is lost than if separate coating applicators are used for applying each of the coating layers. When separate applicators are used, the optical fiber having the primary coating layer thereon must be drawn into the secondary coating applicator and uncured primary coating material may fall away from the optical fiber as it is strung between the applicators. Using a dual applicator eliminates or reduces this problem.
Since the primary and secondary coating layers are applied in a dual applicator, it is not possible to physically obtain access the primary coating layer without removing a portion of the secondary coating layer. Currently, the diameters of the optical fiber and of the secondary coating layer are measured with diameter gauges at the ends of the coated fiber. Measuring these diameters, however, does not provide information about the diameter of the primary coating layer.
It would be desirable to provide a technique for measuring the diameter of the primary coating layer after the primary and secondary coating layers have been applied to the optical fiber without having to physically remove a portion of the secondary coating layer. The primary coating layer must meet certain requirements in order to be deemed satisfactory. One of these requirements is that the primary coating layer must have a proper thickness, or diameter. Another requirement relating to the primary coating layer is that it should have an eccentricity that is within desired or prescribed limits. Therefore, a need exists for a technique for measuring the diameter of the primary coating layer to ensure that it is within desirable or prescribed limits in terms of its diameter and/or in terms of its eccentricity. Furthermore, it would be desirable to perform the technique quickly during the optical fiber cable manufacturing process so that information relating to the diameter of the primary coating layer can be utilized by the draw tower to alter, if necessary, the manufacturing conditions in real time to ensure that the amount of cable having an improper primary coating layer diameter, if any, is minimized or eliminated.
U.S. Pat. No. 5,208,645 to Inoue, et al discloses a method and an apparatus for optically measuring the thickness of an inner coating layer of an optical fiber. The technique disclosed in Inoue, et al. utilizes an observation that, when light is projected through an optical fiber having a coating layer thereon, an intensity distribution having two peaks occurs. It was further observed that these peaks have levels that could be correlated to the thickness of a carbon coating layer on the optical fiber. In order to apply the technique, the thickness of the carbon coating layer is measured by another technique, such as measurement of the electrical resistance of the coating layer. A calibration curve is generated that correlates the electrical resistance to the peaks associated with the intensity distribution. By correlating the peaks of the intensity distribution to the calibration curve, the thickness of the coating layer can be ascertained.
One of the disadvantages of this technique is that it provides no way of determining the thickness of an inner coating layer when more than one coating layer surrounds the optical fiber. Therefore, this technique is unsuitable for determining the diameter of the primary coating layer after the secondary coating layer has been applied. Also, the carbon coating does not correspond to the primary or secondary coating layers of a coated optical fiber, because these layers are comprised of a polymer material. Rather, the carbon coating layer is a coating layer that is applied before the primary coating layer has been applied. Furthermore, the technique utilizes light scattering effects caused by light scattered by the carbon coating to generate the intensity distribution. It does not utilize data relating to light transmitted through the coated optical fiber. Therefore, this technique could not be used to obtain information relating to a coating layer disposed underneath another coating layer.
Accordingly, a need exists for a method and an apparatus for measuring the diameter of the primary coating layer of a coated optical fiber and/or for determining the eccentricity of the primary coating layer.
The present invention provides an optical detection system for determining the diameter of a primary coating layer that has been applied to an optical fiber and/or for determining the eccentricity of the primary coating layer. The system comprises a light source for projecting a beam of light onto the coated optical fiber in a direction substantially perpendicular to the axis of the optical fiber. A lens focuses light passing through the coated optical fiber onto an optical detector. The optical detector is electrically coupled to a signal processor that processes the output of the optical detector and determines the diameter of the primary coating layer and/or the eccentricity of the primary coating layer.
In accordance with the preferred embodiment of the present invention, the method and apparatus of the present invention are incorporated into the optical fiber cable manufacturing process so that if a determination is made that the primary coating layer does not have the proper diameter or eccentricity, the manufacturing process can be adjusted in real time to ensure that the primary coating layer will have the proper diameter or eccentricity on an ongoing basis as the optical fiber cable is being manufactured. However, the method and apparatus of the present invention may also be applied off-line to determine the diameter of the primary coating layer and/or the eccentricity associated with the primary coating layer. For example, utilizing the present invention off-line would enable a purchaser of coated optical fibers to determine whether the diameter of the primary coating layer and/or the eccentricity of the primary coating layer are satisfactory.
In accordance with the preferred embodiment, laser light is projected from a laser onto the coated optical fiber in a direction perpendicular to the axial direction of the fiber, i.e., in a direction perpendicular to the direction of travel of the fiber. A lens positioned perpendicularly to the axial direction of the fiber and parallel to the direction of projection of the laser light receives the laser light that passes through the coated optical fiber and focuses the light onto the optical detector. The optical detector converts the optical signals into electrical signals and outputs the electrical signals to the signal processor, which processes the electrical signals to determine the diameter of the primary coating layer. The signal processor may also process these electrical signals to determine the eccentricity of the primary coating layer.
It has been determined that when light is projected onto an optical fiber having primary and secondary coating layers applied thereto, the image received by the optical detector will contain relatively bright lines corresponding to relatively high intensity at the interfaces between the coating layers. The locations of these lines can be used to determine the diameter of the primary coating layer and/or the eccentricity of the primary coating layer.
Although the eccentricity of the coated optical fiber can be determined using the aforementioned laser, detector and signal processor, an additional setup comprising a second laser, detector and signal processor can be used to ensure that the eccentricity is accurately calculated. A slim possibility exists that, in the case where the primary coating layer is not concentric with the fiber cladding, this eccentricity may be the result of a deformity in a portion of the primary coating layer that is in the direct path from the laser to the detector. This eccentricity may not be ascertainable by the signal processor from the image data. Using the additional setup ensures that this eccentricity will not remain undetected.
In the case where the additional setup is used, the laser and detector of the additional setup will be perpendicular to the first laser and detector and to the coated fiber. The rays of light projected by each of the lasers will be substantially in the same plane, which will be perpendicular to the axial direction of the coated optical fiber. Therefore, if the eccentricity is not detected by one setup, it will be detected by the other setup. The data collected by each of the detectors can be processed by a single signal processor or a separate signal processor may be utilized to process the output of each of the detectors.
These and other features and advantages of the present invention will become apparent from the following description, drawings and claims.